1. Field of the Invention
This invention relates to an aqueous potassium hydroxide dry scrubber method that provides for reduction of acid gases, nitrogen oxides, sulfur oxides, hydrogen chloride and hydrogen fluoride from carbonaceous fuel combustion flue gases. In addition, if an electrostatic precipitator (ESP) is used to remove particulate from the combustion flue gases, its performance will also improve.
2. Description of the Prior Art
U.S. Pat. Nos. 4,246,245; 5,814,288 describe the use of calcium/magnesium hydroxide/oxides in a dry scrubber mode wherein the flue gas is brought near to its dew point to enhance the alkali-sulfur dioxide reactions. The alkalis, either in slurry or dry form are introduced into the flue gas upstream of normally a baghouse to capture the sulfur dioxide as alkali sulfites/sulfates that are collected on the bags and removed from the flue gas stream. While these methods accomplish their intended purposes, they provide only SO2 removal from the flue gas; however, sulfur trioxide (SO3), a fly ash conditioning agent, is also removed. With these technologies, if a downstream ESP is used to collect particulate, the efficiency suffers due to increased fly ash resistivity.
Sodium based compounds have also been used in a dry scrubber mode (e.g. U.S. Pat. Nos. 4,960,445 and 5,002,741). These alkalis are also introduced into the flue gas upstream of an ESP or baghouse. They have proved effective for reducing both sulfur and nitrogen oxide emissions. In addition, ESP performance is improved. Whereas the sodium based compounds are effective in reducing sulfur and nitrogen oxide emissions and improving ESP performance, they have no commercial value. Further, sodium based compounds are not desirable in landfills for they are soluble and can enter underground aquifers to increase water salinity. Although sodium sorbents are very effective at reducing air pollutants, the potential ground water pollution with the use of these sorbents can offset their air pollutant reduction benefits.
There are many types of devices to reduce sulfur dioxide emissions to the levels prescribed by the U.S. EPA, but the same cannot be said about nitrogen oxides. In the Year 2003, the U.S. EPA will regulate nitrogen oxides emissions for all types of coal-fired boilers to 0.15 lb NOx/106 Btu in the Eastern and Midwestern States during the ozone season (May through September).
Most commercial NOx reduction technologies cannot meet this limit. Although technologies are in the developmental stage, the only technology available today that will guarantee such a low level of NOx emissions is the Selective Catalytic Reduction (SCR) technology. The SCR method uses ammonia addition and a downstream catalyst placed in the flue gas stream to destroy the NOx produced in the coal combustion process. This approach is expensive both from capital and operating cost perspectives. Further, arsenic in the coal ( less than 10 ppmw) can poison the catalyst, shortening its life. Still further, ammonium sulfites/sulfates and calcium sulfates from the combustion process can blind the catalyst, reducing its effectiveness. The U. S. EPA also regulates particulate matter at sizes less than 2.5 microns (PM2.5). SCR technology requires ammonia addition and there is always some ammonia slip present to react with sulfur dioxide (SO2) and nitrogen dioxide (NO2) to increase fine particulate [(NH4)2SO4 and NH4NO3] concentrations in the atmosphere (PM2.5).
The method of the present invention provides the benefits seen with the use of sodium sorbents but rather than creating landfill ground pollution problems, provides a potassium sulfate/nitrate/fly ash mix that has considerable fertilizer value (the K2SO4/KNO3 value estimated at $150/ton by agricultural engineers). The potassium hydroxide spray-dry scrubbing technique can be used as a NOx trim in combination with low NOx burners and Reburning technology to bring coal-fired power plants into NOx emission compliance, thus providing an option to the SCR technology.
It is also the low cost method, when used in combination with a baghouse to bring small-scale coal-fired stokers (stokers up to 50 million Btu/hr) into SO2, NOx and particulate compliance without the need for any additional SO2/NOx control techniques. Today, small-scale coal-fired stokers are facing elimination due to the more stringent state environmental regulations. The only solution currently available today is to replace coal-fired stokers with relatively expensive fluidized bed coal combustion systems wherein sulfur dioxide and nitrogen oxides may be controlled and baghouses are included for particulate control. Thus, many small-scale stoker users are switching to lower capital cost natural gas fired boilers/hot water heaters. Although low in capital cost, the switch to natural gas drives production/manufacturing costs up due to the higher price of natural gas compared to coal that increases annual operating costs. What is needed for these coal-fired stokers is a low capital and operating cost retrofit technology that reduces nitrogen and sulfur oxides and particulate from small coal fired stoker units and that is what the potassium hydroxide dry scrubber provides.
I have discovered a process using an aqueous solution of potassium hydroxide to reduce acid gases; nitrogen oxides, sulfur oxides, hydrogen chloride and hydrogen fluoride from carbonaceous fuel combustion flue gas. In addition, if an electrostatic precipitator (PSP) is used to remove particulate from the combustion flue gases, its performance will also improve. The application of the technology preferably comprises adding a co-current flue gas-spray tower upstream of an ESP or baghouse. Aqueous potassium hydroxide (KOH) is spray dried into the flue gas upstream of the particulate control device. The KOH reacts with SO2 and SO3 to form K2SO4, NO and NO2 to form KNO3, HCL to form KCl and HF to form KF. These salts are captured as particulate and removed with the carbonaceous-fuel fly ash from an ESP or baghouse.